A funny thing happens if you try to download the Opera browser for Mac OS X from the main page : 1. Click on the button “Download Opera for Mac OS X”: 2. In the window that opens, we look at what we are offered to download: We offer to download the great Opera installer for Windows :) Such a thing happens in both Mozilla Firefox and Apple Safari.
To download Opera for Mac, I eventually had to go through this link: http://www.opera.com/browser/download/ and only then the correct installation package was downloaded.